Method for structural health monitoring using a smart sensor system

ABSTRACT

The structural health monitoring method of the present invention utilizes ultrasound to determine information about deformation, stress and/or damage in structural elements. The method propagates ultrasound through at least a portion of a material having fully-reversible nonlinear elasticity, receives the ultrasound which has been propagated through at least a portion of the material and determining information about the structural element from attenuation and/or time of flight of said received ultrasound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/130,234, filed on May 30, 2008, which in turn is a non-provisional ofU.S. provisional application No. 60/941,864, filed on Jun. 4, 2007, thedisclosures of which are hereby incorporated by reference in theirentirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was reduced to practice with Government support underGrant No. DAAD19-03-1-0213 awarded by Army Research Office; theGovernment is therefore entitled to certain rights to this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for structural healthmonitoring (SHM) using a sensor system and ultrasound. The sensor systemmay be used to monitor the structural health of structures includingcivil engineering structures, such as bridges, buildings and underwaterstructures, critical structural elements in the automobile, aerospaceand petrochemical industry as well as storage structures and reactors.

2. Description of the Related Technology

SHM is used to maintain and preserve the structural integrity ofstructures, which degrade over time from exposure to excipient factors,such as earthquakes, storms, pollution, vibration, traffic, and otherenvironmental factors. In the last few decades there has been tremendousinterest in developing methods and sensors, such as strain gages,displacement sensors, accelerometers, magneto-strictive sensors, fiberoptic sensors and piezoelectric sensors, for detecting structuraldegradation or damage.

Current SHM techniques utilize either global sensing methods or localsensing methods. Global dynamic methods excite a structure using lowfrequency acoustic waves and detect the resulting corresponding naturalfrequencies of the structure. The natural frequency data may then bemanipulated with various algorithms to locate and quantify damage insimple structures. Global dynamic methods, however, rely on a relativelysmall number of low order modes that are insufficiently sensitive todetect localized incipient damage, which may be critical to structuralintegrity. Additionally, the application and detection of low frequencyexcitation, typically below 100 Hz, is easily contaminated bysurrounding vibrations and noise. Global static methods, such as staticdisplacement response and static strain measurement, are alsoimpractical since they are too expensive to enable a cost and timeefficient structural evaluation.

Local sensing methods, such as ultrasonic wave propagation techniques,acoustic emissions, magnetic field analysis, electrical methods, dyepenetrant testing, impact echo testing and X-ray radiography, are alsoproblematic. A common limitation of local sensing methods is that aprobe needs to be moved around the structure to first identify apotential site of structural damage if the location of structuralweakness is not already known. Attempts to overcome this difficulty,with varying success, included measuring the response from an array ofpiezoelectric patches on the surface, magneto-elastic sensors and fiberBragg grating methods.

Of the various local sensing methods, ultrasonic wave propagation is oneof the most promising, enabling detection of damage and structural flawswith a high degree of sensitivity. Examples of ultrasonic wavepropagation are disclosed in U.S. Pat. No. 6,996,480 and Lars Lading, etal., “Fundamentals for Remote Structural Health Monitoring of WindTurbine Blades”, Riso National Laboratory, 2002. The main drawback ofthe ultrasonic method is that it requires several transducers to beinstalled at various locations to monitor a particular structure due tothe attenuation and absorption of sound waves in these structures.Often, ultrasonic transducer installation is time-consuming andexpensive making such methods impractical.

Ultrasonic methods also typically require complex data processing. Inaddition to being expensive, ultrasonic methods also render thestructure unavailable for use throughout the duration of the test. Dueto the nature of sound waves, excitation means for the ultrasonictransducers has to be coupled directly onto the structure beingmonitored. In addition, such systems typically only work over relativelynarrow temperature ranges and under limited environmental conditions.

In spite of recent innovations, as far as the inventors are aware, nosensor, to date, enables highly sensitive detection of various types ofdeformation under a wide range of variable atmospheric, corrosive andtemperature conditions. Current sensors additionally require complexdata processing and large amounts of information to analyze structuraldeformation. Therefore, there is a need to develop a sensor systemcapable of extracting important parameters from minimal amounts of datausing simple data processing techniques and which is further capable ofhighly sensitive detection irrespective of environmental conditions.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method for structuralhealth monitoring using ultrasonic wave propagation through a systemincluding one or more smart sensor elements comprising at least onematerial which exhibits fully-reversible nonlinear elasticity (FRNE).

Another aspect of the invention is directed to a method comprisingoperatively associating at least one smart sensor element with astructure, propagating ultrasound through the smart sensor element,receiving the ultrasound propagated through the smart sensor element,and determining information relating to the structure from theattenuation and/or the speed of the received ultrasound. The smartsensor element comprises at least one material which exhibitsfully-reversible nonlinear elasticity (FRNE).

Another aspect of the invention is directed to a sensor systemcomprising at least one smart sensor, an ultrasound emitter and areceiver for receiving ultrasound propagated through the sensor.

Another aspect of the invention is applicable if the structuralcomponent itself, is made of a material that exhibits fully-reversiblenonlinear elasticity (FRNE), in which case it can be monitored directlyusing ultrasound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic showing the formation of incipient kink bands.

FIG. 1( b) is a schematic showing mobile dislocation walls.

FIG. 1( c) is a schematic showing permanent kink bands.

FIG. 2 is a schematic diagram of a complex loading unloadingstress-strain response of a kinking nonlinear elastic solid. The arrowsin the figure represent the loading direction.

FIG. 3 is a graph of

$\frac{c\; 44}{c\; 33}$

vs. c/a.

FIG. 4 is a schematic of an ultrasonic sensor system in accordance withthe present invention.

FIG. 5 is a graph of fully reversible hysteretic stress-strain behaviorof Ti₃SiC₂.

FIG. 6 is a graph of ultrasound attenuation versus stress forcoarse-grain Ti₃SiC₂ under load.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one aspect, the invention is directed to a method for structuralhealth monitoring (SHM) of materials using ultrasound and a smart sensorsystem. The method involves operatively associating at least one smartsensor element 1 including at least one material which has afully-reversible nonlinear elasticity (FRNE)³, with a surface of astructure or structural element 2 and using an ultrasonic transducer topropagate ultrasound through the smart sensor element 1. As thestructure 2 degrades over time, structural stresses will be physicallytransferred to the sensor 1. By monitoring and recording changes in theultrasound attenuation and/or time of flight of ultrasound propagatedthrough sensor 1 by means of ultrasonic transducer 3, it is possible todetermine one or more of the presence of, location of and severity ofdamage to the structure, as well as generate an image of the structuraldamage by using a properly spaced array of sensors 1.

Sensor 1 should be associated with the structural element 2 in a mannerwhereby deformation of structural element 2 causes a correspondingdeformation of sensor 1. Thus, it is often desirable to have a surfaceof sensor 1 maintained in direct contact with a surface of structuralelement 2 for this purpose. Any suitable means for maintaining sensor 1in association with structural element 2 may be employed. In oneembodiment, sensor 1 may be bonded to structural element 2. This bond issufficiently secure so as to essentially prevent relative movementbetween a sensor 1 and structural element 2 to thereby provide excellenttransmission of stress from structural element 2 to sensor 1. In anotherembodiment, sensor 1 may be clamped to structural element 2.

One aspect of the SHM method and the sensor system of the presentinvention involves selecting an appropriate material for use in sensor1. The material should have FRNE characteristics to enable sensor 1 torecord and reconstruct the applied stress and deformation of a structureover time. This deformation phenomenon may be partially attributed tothe reversible formation and the disassociation of incipient kink bands(IKB). Such materials are sometimes referred to as kinkingnonlinear-elastic materials or KNE materials.

FIG. 1( a) shows that as a KNE material is stressed, IKBs havingapproximately parallel walls of opposite sign dislocations that are notdissociated, i.e. still attracted to each other at their ends, areformed prior to the generation of regular kink bands (KB). The IKBsdissociate when the load is very high and produce mobile dislocationwalls (MDW), shown in FIG. 1( b), which lead to permanent deformation.Once the MDW coalesce, kink boundaries are formed and subsequentlyproduce the kind band structure depicted in FIG. 1( c). —It is importantto note that this invention is not restricted to the micromechanismsshown in FIG. 1. Any solid in which fully reversible dislocation motionoccurs—such as dislocation pileups in plastically anisotropic solids—canbe used for this invention. The key aspect is fully reversibledislocation motion.

The characterizing feature of KNE material deformation suitable for thesensor elements of the present invention is the formation of fullyreversible, rate-independent, closed hysteresis loops in stress-straincurves, delineates deformation process described in FIG. 1( a)-1(c).These loops are strongly influenced by grain size, with the energydissipated per unit volume per cycle, W_(d), being significantly largerin the coarse-grained material. As the stresses become larger, thehysteresis loops become larger until the structure fails. FIG. 2demonstrates a complex loading and unloading stress-strain response ofKNE solids, which are stress memory materials capable of remembering thehighest and/or lowest points of these hysteresis loops or cycles.

KNE solids are further characterized by plastic anisotropy, whichtypically occurs in materials having high c/a ratio, and/or acomplicated multi-atom unit cells. Plastic anisotropy only allows fordeformation by slip on one easy slip system, which for hexagonal solidsis basal slip. Dislocation motion on other slip systems is verydifficult. The plastic anisotropy due to high c/a ratio only allows fordislocations on one easy slip system. Any other kinds of dislocationsare of extremely low probability and thus are insignificant.

FIG. 3 identifies materials capable of kinking non-linear deformation bygraphing c₄₄/c₃₃ versus c/a. KNE solids having a large c/a ratio lie tothe right of the vertical line. Materials having a c/a ratio of aboveabout 1.2 may be suitable for use in the present invention if theyexhibit fully-reversible nonlinear elasticity. More preferably,materials having a c/a ratio above about 1.5 are employed. Solids thatallow for more than one slip system are typically unsuitable forfabricating the sensor 1 of the present invention. KNE solids thatexhibit FRNE should thus have a c/a ratio of at least 1.2, morepreferably, the c/a ratio should be at least 1.5. There is no upperlimit on the c/a ratio since the higher the c/a ratio, the better thematerial will perform as a sensing element.

In one embodiment, sensor 1 is constructed from at least one KNE solidthat is stable over a temperature range of about 4 K to about 1000 K,more preferably over a temperature range of about 77 K to about 1000 Kand most preferably over a temperature range of about 123.15 K to about973.15 K. In another embodiment, the KNE solid is chemically stable,inert and resistant to aggressive environmental and atmosphericconditions. More preferably, the KNE solid is generally corrosionresistant, and most preferably, the KNE solid is specifically resistantto acidic, basic, salt containing and other corrosive atmospheres suchas S-containing ones.

Any material exhibiting FRNE behavior and have a c/a ratio above about1.2, preferably, above about 1.5 can be used as a sensor materialdepending on the maximum possible stress, temperature and ambientconditions that the material can withstand without undergoingsignificant alterations. Sensor 1 may be configured in various shapes toincrease the working range of stress and/or strain according to therequirements of the structural element 2.

In an exemplary SHM method of the present invention, at least one sensor1 containing a material exhibiting FRNE behavior is clamped to a portionof a structural element 2. Optionally, the method includes a preliminarystep of identifying a defect, damage or stress within structural element2 or identifying a particular portion of structural element 2 for whichmonitoring is desirable, using any standard technique. Sensor 1 can beemployed in conjunction with a suitable system, to detect and imagestress and damage within structural element 2. Alternatively, sensor 1may be used in a sensor system to detect inherent defects in structuralelement 2.

In one embodiment, a conventional ultrasound sensor can be used tomeasure the inherent/existing defects in structural element 2 prior toinstallation of the sensing system of the present invention. Afterinstallation of the sensing system of the present invention, the sensingsystem will measure how the existing inherent defects grow or change bydetecting the changing stress-state in the structural element 2.

As shown in FIG. 4, sensor 1 may be associated with a structural element2 by bonding or using a clamp or other suitable means such thatstructural deformation and stress can be effectively transmitted fromstructural element 2 to sensor 1. The securing means should also enablesecure long-term retention of sensor 1 on structural element 2. In apreferred embodiment, hardened steel clamps can be employed forassociating sensor 1 with structural element 2 for application inbridges or civil structures; and the clamp can be a hard ceramic if theapplication temperature is high and/or the atmosphere is corrosive.

Sensor 1 may be calibrated by propagating an ultrasonic pulse throughsensor 1 to obtain an initial calibration of the state of sensor 1 foruse as a baseline to monitor the state of structural element 2. Asstructural element 2 deforms, is stressed or degrades over time, thedeformation and/or stress will be physically transferred from structuralelement 2 to sensor 1. Ultrasonic pulses may be periodically propagatedthrough sensor 1 to obtain measurements and monitor the condition ofstructural element 2 relative to the baseline condition measured duringcalibration of sensor 1.

It is the intention that the ultrasonic pulses should pass solelythrough sensor 1 and not penetrate structural element 2 so as to obtainmeasurements of the attenuation and time of flight of the ultrasonicpulses, as influenced by sensor 1 only. A transducer 3 optically coupledto sensor 1 emits ultrasonic pulses and may be used to receive theultrasonic pulses after they have passed through at least a portion ofsensor 1. The collected data may be recorded by a data storage unitassociated with a data processing unit 4. Data processing unit 4 cangenerate an accurate image of deformation, stress and/or damage tostructural element 2 in real time. Data processing unit 4 may alsodetermine the location of deformation, stress or damage based on time offlight data as well as determine the severity of structural damage usingultrasonic attenuation by sensor 1. In a preferred embodiment, data maybe transmitted wirelessly to data processing unit 4. Over time it ispossible to generate a history of the changes in structural element 2.

The SHM method and sensor system of the present application are capableof providing significant information regarding structural damage, stressand deformation to structural elements using a minimal amount of data byinterpolating the deformation history between two consequentmeasurements. Furthermore, the sensor system is capable of measuring awide range of stresses and/or deformation while requiring a relativelyminor amount of data processing.

The SHM method and sensor system of the present invention function bycapturing ultrasound attenuation, which is used to determine thenonlinear elasticity or reversible dislocation motion within sensor 1.When sensor 1 experiences stress or deformation transferred fromstructural element 2, IKBs and/or reversible dislocations are nucleatedand interact with the ultrasound pulses, causing attenuation andinfluencing time of flight of the pulses. Upon unloading, the reversibledislocations or IKBs annihilate and the attenuation and time of flightinfluence is no longer observed. This reversible behavior of stress,ultrasound attenuation and influence on time of flight may be used todetermine the induced stress for every point of the hysteretic loop,shown in FIG. 2. Furthermore, the ultrasound attenuation may also beused to determine the maximum stress structural element 2 experiencedbefore failure and to deduce the stress/strain deformation history ofstructural element 2.

The sensor system of one embodiment of the present invention may beuseful for structural health monitoring of numerous structures,particularly civil engineering structures such as bridges, buildings andunderwater structures; structural elements of automobiles, trains,aircraft, aerospace devices, watercraft, submersibles and other man madedevices and machines; and reactors, storage structures, etc., thatdegrade with time due to exposure to excipient factors, such asearthquakes, storms, pollution, vibration, traffic, and otherenvironmental factors. The SHM methods and sensor systems of the presentinvention enable an accurate determination of the integrity of thesestructures, relative to baseline integrity, at any point in time, andmay enable engineers to determine when and where structural repair isnecessary. The method of the present invention is equally applicable tohistorical as well as modern structures and may be used to maintain andpreserve structural elements, monitor structural damage over time andwarn of impending structural failure. It is envisioned that the SHMmethod of the present invention may be particularly beneficial to theaerospace, automobile and petrochemical industries.

An example of KNE solids are materials having M_(n+1)AX_(n)- orMAX-phases, where M is an early transition metal, A is an A-groupelement, X is carbon and/or nitrogen, and n=1-3. The MAX phases,numbering over 50, are ternary carbides and nitrides. The crystalstructure of MAX phases comprise hexagonal nets of “A” atoms separatedby three nearly close-packed “M” layers that accommodate “X” atoms inthe octahedral sites between them. Typically, suitable materials aresolid, crystalline materials with a crystal lattice structure. Materialshaving high temperature capabilities such Ti₂AlC, are particularlyuseful since such materials will be better able to withstand significantatmospheric temperature variations in use.

In another embodiment, the actual structural component is fabricatedwith a material exhibiting fully-reversible nonlinear elasticity. Inwhich case, the entire part can be used to monitor its health and allthat required is to propagate ultrasound through the entire or parts ofthe structure and measuring its attenuation.

Of the MAX phase compounds, Ti₃SiC₂ and Ti₂AlC are some of the mostpromising, lightweight candidate materials for use in sensing elementssuitable for high temperature structural monitoring and otherapplications. Despite having a density of about 4.5 gm/cm³, Ti₃SiC₂ andTi₂AlC have a stiffness about three times as high as titanium, but areas readily machinable as titanium. With a Vickers hardness ofapproximately 3 GPa, they are relatively soft, unusually resistant tothermal shock and highly damage tolerant. Unlike most brittle solids,edge cracks do not emanate from the corners of hardness indentations.Rather, intensive kinking, buckling and bending of individual grainstake place in the vicinity of the indentations, resulting inpseudo-plastic behavior over a wide range of temperature.

Polycrystalline Ti₃SiC₂ are further capable of being cyclically loadedin compression at room temperature to stresses up to 1 GPa, and fullyrecover on the removal of the load, while dissipating about 25% (0.7MJm⁻³) of the mechanical energy, as shown in FIG. 5. These loss factorsare higher than most woods, and comparable to polypropylene and nylon.FIG. 5 depicts the typical behavioral plot of a structure composed ofcoarse-grained Ti₃SiC₂. The stress-strain curve delineates a fullyreversible, rate-independent, closed hysteresis loop characteristic ofKNE solids. Furthermore, Ti₂AlC, graphite, hexagonal-boron nitride, mostof the hexagonal metals and mica have similar deformation behavior,which can be attributed to the reversible formation of dislocations andcan be used to fabricate sensing elements in accordance with the presentinvention if the particular material is suitable for the stress,temperature and atmospheric conditions to which it will be subjected inuse.

The behavior of the KNE materials allows use of a calibration curvesimilar to that shown in FIG. 6, to correlate ultrasound attenuation bythe KNE materials with the stress exerted on sensor 1. In addition,since KNE materials exhibit reversible deformation but provide adifferent response as a result of such reversible deformation,additional information such as the prior maximum deformation anddeformation history can be obtained from the ultrasound attenuation dataobtained from the KNE materials. This can be seen in, for example, FIG.6, where, upon reduction of the applied stress on the KNE material, theattenuation followed a different curve (downward arrow) than wasfollowed during application of the applied stress (upward arrow) to theKNE material.

This behavior of the KNE material also permits imaging of thedeformation, stress or damage to a structural element since theinformation required for such imaging can be obtained by comparison to acalibration curve and/or via application of simple algorithms tocorrelate ultrasound attenuation and/or time of flight with specificstructural stress and/or damage in the structural element.

References cited herein are listed below and the disclosures of thelisted references are hereby incorporated by reference in theirentirety:

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1. A method for monitoring a structure comprising: propagatingultrasound through at least a portion of the structure which exhibitsfully-reversible nonlinear elasticity; receiving the ultrasound after ithas propagated through said portion of the structure which exhibitsfully-reversible nonlinear elasticity, and determining informationrelating to said portion of the structure which exhibitsfully-reversible nonlinear elasticity from at least one of attenuationand time of flight of said received ultrasound.
 2. The method of claim1, wherein said method is capable of being carried out within atemperature range of about 123.15 K to about 973.15 K.
 3. The method ofclaim 1, wherein said portion of said structure which exhibitsfully-reversible non-linear elasticity comprises a material selectedfrom the group consisting of materials having MAX phases.
 4. The methodof claim 3, wherein the portion of the structure having fully-reversiblenon-linear elasticity comprises a material selected from the groupconsisting of Ti₃SiC₂, Ti₂AlC, graphite, hexagonal-boron nitride, mica,and hexagonal metals.
 5. The method of claim 4, wherein the hexagonalmetal is selected from the group consisting of Co, Mg, and Ti.
 6. Themethod of claim 1, further comprising a step of identifying a site ofdeformation, stress or damage in the structure prior to said step ofpropagating ultrasound through at least a portion of the structure. 7.The method of claim 1, wherein said method provides a maximum stresssaid structure experienced before a potential occurrence of structuralfailure
 8. The method of claim 1 wherein said method provides adeformation history of said structure.
 9. The method of claim 1, whereinsaid method provides an image of deformation, stress or damage to saidstructure.
 10. The method of claim 1, wherein said information aboutsaid structure is determined from attenuation of said ultrasound by saidportion of the structure.
 11. The method of claim 1, wherein saidinformation about said structure is determined from time of flight ofsaid ultrasound through said portion of the structure.
 12. The method ofclaim 1, wherein said portion of the structure which exhibitsfully-reversible nonlinear elasticity comprises a material having a c/aratio of at least 1.2.
 13. The method of claim 1, wherein said portionof the structure which exhibits fully-reversible nonlinear elasticitycomprises a material having a c/a ratio of at least 1.5.
 14. The methodof claim 1, wherein said ultrasound is propagated through an entire partof said structure.
 15. The method of claim 1, wherein said ultrasound ispropagated through said entire structure.
 16. The method of claim 1,wherein said portion of the structure which exhibits fully-reversiblenonlinear elasticity comprises a material selected from the groupconsisting of materials of the formula: M_(n+1)AX_(n), where M is anearly transition metal, A is an A-group element, X is carbon and/ornitrogen, and n=1-3.
 17. The method of claim 16, wherein said portion ofthe structure which exhibits fully-reversible nonlinear elasticitycomprises a material selected from the group consisting of ternarycarbides and ternary nitrides.